No. G35] THE HAIR OF MAMMALS 497 



wraps the papilla (Fig. 178). The shaft elongates up- 

 ward, and emerges through the epidermis, an aperture 

 thereafter known as the mouth of the follicle, and con- 

 tinues to grow, the growth being exclusively confined to 

 the bulbous lower, or proximal portion of the shaft. 

 Here the conversion of matrix cells into keratinized hair 

 shaft cells continually progresses. Mammal hairs are in 

 general either circular or elliptical in cross section. 2 

 Those which are circular are straight, or but slightly 

 curved, while those of elliptical cross section are curly 

 or kinky, the amount of curl being dependent upon the 

 flatness of the ellipse. 



The hair shaft consists of four structural units (Figs. 

 167 and 168) : (1) the medulla, sometimes termed the 

 pith, from a somewhat analogous structure in plant 

 stems, and which is built up of many shrunken and vari- 

 ously disposed cells or chambers, representing dried and 

 cornified epithelial structures connected by a branching 

 filamentous network, which sometimes completely fills 

 the medullary column, but which is interrupted in many 

 cases ; (2) the cortex, or shell of the hair shaft, surround- 

 ing the medulla, and composed of elongate, fusiform cells 

 or hair-spindles, coalesced together into a horny, almost 

 homogeneous, hyaline mass and forming in many cases, 

 where the medulla is reduced, a large proportion of the 

 hair shaft: ('.'!) the puimcnf firatmlcs, to which the color 

 of the hair is primarily due (though in some hairs the 

 pigment is diffuse and not in granular form), scattered 

 about within or between the hair spindles, and in some 

 hairs arranged in definite patterns; and (4) the cuticle, 

 or outermost integument of the hair shaft, lying upon 

 the cortex, and composed of imbricated, thin, hyaline, 

 colorless scales of varying forms and dimensions. It is 

 the forms, relationships, and measurements of these four 

 elements, together with the measurements of the diam- 



